1. Field of the Invention
The present invention relates to a DC-DC converter circuit.
2. Description of the Related Art
A known DC-DC converter circuit (DC-DC converter) converts an input direct-current voltage to a direct-current voltage of a different value and outputs it as a power supply voltage to a later-stage circuit. The DC-DC converter circuit includes an integrated circuit (IC) which constitutes a control center thereof.
Furthermore, a known digital-to-analog converter device (D/A converter) includes a logic buffer equipped with a chip-enable terminal that sends an output to an amplifier with an output of the D/A converter as the input, and a control circuit that sends a disenable signal to the chip-enable terminal during a period when the power supply is unstable and that sends an enable signal to the chip-enable terminal in a state in which the power supply is stable (see Japanese Patent Application Laid-Open Publication No. H7-264066).
Furthermore, a known DC-DC converter has a main switching element that is not turned on immediately by the control circuit IC outputting pulse signals, and after a certain period of time has elapsed, the main switching element performs output, such that when the output voltage of the main switching element does not reach the threshold value at a minimum pulse width, the DC-DC converter prevents incomplete output voltage with respect to the rated output voltage that normally occurs at this point or overshoot that occurs in this case (see Japanese Patent Application Laid-Open Publication No. 2003-244942).
In electrical appliances which have a built-in DC-DC converter circuit such as that described above, when a power cord (AC cord) is connected to a commercial alternating-current power supply, the voltage input to the DC-DC converter circuit via a power supply circuit which executes AC-DC conversion rises over a specified time from 0 V to reach a substantially constant value. In terms of design, the DC-DC converter circuit has something called a minimum operating voltage value set for the input voltage, and the input voltage must be the minimum operating voltage value or greater in order for the IC to operate. Meanwhile, during the time from when the AC cord is connected to the commercial alternating-current power supply until the input voltage to the DC-DC converter circuit stabilizes at a substantially constant value, there are cases in which the input voltage becomes unstable near the minimum operating voltage value due to the effect of a load or the like on the later-stage circuit, and in such cases, the IC may end up operating before the input voltage to the DC-DC converter circuit stabilizes, making it possible for an IC malfunction, disordered output values from the DC-DC converter circuit, and the like to occur. Note that for each of the known devices described above, the structure is insufficient to eliminate the instability of the DC-DC converter circuit caused by instability of the input voltage which can occur when connecting to such an external commercial alternating-current power supply.